Here \(\delta =\pm 1\) , \({{e}_{x,y}}=\left( 0,{{\mathbf {e}}_{x,y}} \right)\)  and \(k=\left( \omega ,\mathbf {k} \right)\)  are 4-polarization vectors and the 4-momentum of the photon of the external field, and: \({{k}^{2}}=0,\ e_{x,y}^{2}=-1,\ {{e}_{x,y}}k=0\) . This is a second-order process with respect to the fine structure constant and is described by two Feynman diagrams (see Figure REF ). The wave functions of the electron are determined by the Volkov functions [1]}, the intermediate states of the electron are given by the Green function in the field of a plane light wave (REF ) [2]}, [3]}. The amplitude of such a process after simple calculations can be represented in the following form (see, for example, [4]}, [5]}, [6]}):
\( S_{fi}=\sum _{l=-\infty }^{\infty } S_l,\) 
